1. Field of the Invention
The present invention relates to a manufacturing method of a printed circuit board, particularly, relates to a manufacturing method of filling up appropriately a blind hole formed on a printed circuit board with insulative thermosetting resin
2. Description of the Related Art
Presently, a printed circuit board is required to have high packing density in accordance with development of minimizing electronic apparatuses and equipment. A multilayer printed circuit board having multilayers of wiring boards has been realized for the purpose of satisfying the demand therefor. Each wiring board of each layer is electrically and mechanically connected to each other by way of a through hole and a blind hole (or referred to a non-through hole). A blind hole is filled with insulative resin or conductive resin in accordance with usage of the blind hole, and the resin is hardened.
In a process of manufacturing a multilayer printed circuit board, it is required that the blind hole be filled efficiently and securely with resin.
Various kinds of examinations have been performed for the purpose of filling a blind hole with resin efficiently and securely. Japanese Patent Laid-open Publication No. 7-249866/1995, for example, discloses a method of filling a through hole or a blind hole provided on a substrate with hardening paste. According to the method, the filling process is completed by supplying vibration to the hardening paste, which has been temporarily filled up in a through hole or a blind hole provided on a substrate by printing process.
Japanese Patent Laid-open Publication No. 9232758/1997 discloses another filling method. According to this filling method, while manufacturing a wiring board having a blind hole by way of a process of filling up the blind hole with insulative resin in conjunction with coating the insulative resin on the surface of the wiring board, the blind hole is filled up with the insulative resin by coating the insulative resin after the inner circumference of the blind hole has been moistened with a solvent.
Each method mentioned above is an excellent method for filling up a blind hole with insulative resin However, these methods can not sufficiently remove a bubble possibly remaining in the resin filled up.
FIG. 4 typically shows a bubble 20 generating in a blind hole 6 of a printed circuit board. In FIG. 4, the blind hole 6 is formed in a resin layer 3, which is coated on a substrate 1 and hardened. The bubble 20 may generate in a resin 7 by chance, while the resin 7 has been filled up in the blind hole 6 and hardened.
As mentioned above, if a bubble remains in an insulative resin after the insulative resin has been filled up in a blind hole and hardened by heating, the bubble expands and causes a problem in that the resin 7 easily swells when the printed circuit board is exposed to high temperatures such as occurs in a soldering process thereafter. The same problem occurs if the blind hole is not sufficiently filled up with the resin.